How relevant are theories of information to practical library work? Using sociology and the philosophy of social sciences, this essay discusses how theoretical work defines an area of knowledge in relation to other fields, the identification of researchable phenomena, and how phenomena are established as its own subject matter. Theorizing is a generic activity, highlighting connections between theory and practice, profession and field. While proposing that theorizing is important, this paper explores the nature of theorizing in LIS, and suggests that its cognitivistic foundations impede theory development. Information is a linkage between theory and practice, and this paper argues that assessing the forms and coherence of theories consolidates the profession/field nexus. This paper revisits the "segmentation thesis" (Bucher and Strauss, 1961) to characterize the relationship between Library & Information Studies (LIS) and librarianship.